Gems are simple, (usually) lightweight items that are found in the dungeon. They come in three varieties: valuable gems, pieces of worthless glass which look similar, and gray stones. Another type of item that uses the same glyph is the rock, which is dealt with differently.

Valuable (identified) gems may be sold for relatively large amounts of money at most general stores, which makes them valuable as a compact form of money. However, shopkeepers will always buy unidentified gems as if they are worthless glass, and will always sell unidentified gems as if they are valuable, possibly subject to a further 33% surcharge for being unidentified. Valuable gems may also be thrown to a co-aligned unicorn to get a large Luck increase. Throwing any valuable or worthless gem to a unicorn will pacify them, and will variably raise or lower luck in certain cases. See the unicorn page for details. Valuable gems also give score, but as score does not normally matter, this is typically of low importance.

Glass and rocks are relatively worthless, though they can be used for throwing if you should wish to conserve your other attacks. Additionally, they can occasionally be polymorphed into more valuable gems if included in a polypile, though it's not worth wasting a wand charge on them. Rocks can also be changed in to meatballs, useful for taming and for sale in shops.

Despite the name, a worthless piece of <color> glass will not shatter like other glass items if thrown against a wall or dropped while levitating. You can fire them from slings as a lighter version of rocks; like rocks, they can be multishot. If they are used as ammo however, both worthless pieces of glass and precious gems may disappear (like arrows).

Gray stones are a catchall for four different objects until they are identified.

Touchstones may be used to identify glass from gems, and even the variety of gem if it is a blessed touchstone or you are a gnome or archeologist.

Luckstones will augment your luck rather impressively; if you have a blessed luckstone and no other luckitems, your good Luck will not timeout, bad Luck will, and you'll get +3 extra Luck.

Loadstones are a pun (dating back to early editions of Dungeons and Dragons): at 500 weight units, they weigh 50 times as much as the other gray stones, and they autocurse (and are generated cursed) so they cannot be dropped.

Flint stones have no particular purpose, but they do slightly better as sling ammo.

Tables of gems

By value

Some gems' appearances can change. In these cases all the options are listed. The actual appearance is randomly chosen from the options at the beginning of each new game, and is consistent throughout that game (if one fluorite is white, all fluorites in that game will be white). The hardness of a gem can be tested by #engraving with it; if the gem is hard, the game will prompt "What do you want to engrave?"; if it is soft, "What do you want to write in the dust?"

By color

The gems that have a randomized appearance are marked with an asterisk, "*".

Color

Possibilities

Cost

Unidentified sell price

Level

Hardness

0

1

2

*White

dilithium crystal

4500

4

4

6

27

soft

diamond

4000

5

5

3

24

HARD

opal

800

5

7

3

1

soft

fluorite stone*

400

4

7

4

1

soft

worthless glass

0

8

6

4

1

soft

*Blue

sapphire

3000

8

3

6

15

HARD

turquoise stone*

2000

5

6

5

6

soft

aquamarine stone*

1500

7

3

3

1

HARD

fluorite stone*

400

4

7

4

1

soft

worthless glass

0

3

7

5

1

soft

*Red

ruby

3500

6

6

4

21

HARD

garnet stone

700

7

4

5

1

soft

jasper stone

500

3

6

3

1

soft

worthless glass

0

4

3

6

1

soft

*Yellowish brown

amber stone

1000

8

4

4

1

soft

topaz stone

900

3

5

5

1

HARD

worthless glass

0

5

4

3

1

soft

*Orange

jacinth stone

3250

7

7

5

18

HARD

agate stone

200

6

4

6

1

soft

worthless glass

0

6

5

4

1

soft

*Yellow

citrine stone

1500

6

7

6

3

soft

chrysoberyl stone

700

6

3

4

1

soft

worthless glass

0

7

6

5

1

soft

*Black

black opal

2500

3

4

3

12

HARD

jet stone

850

4

6

6

1

soft

obsidian stone

200

5

3

5

1

soft

worthless glass

0

8

7

6

1

soft

*Green

emerald

2500

4

5

4

9

HARD

aquamarine stone*

1500

7

3

3

1

HARD

jade stone

300

7

5

3

1

soft

fluorite stone*

400

4

7

4

1

soft

turquoise stone*

2000

5

6

5

6

soft

worthless glass

0

3

3

3

1

soft

*Violet

amethyst stone

600

8

5

6

1

soft

fluorite stone*

400

4

7

4

1

soft

worthless glass

0

4

4

4

1

soft

Generation and identification

To be formally identified means that a gem can be sold at a shop using its base price instead of just a few zorkmids; even if you've found all the other members of the white gem family, including the worthless glass bauble, the shopkeeper will not take "a white gem called diamond" at face value. Much like other forms of identification, gems can be formally identified by:

scrolls or spells of identification, the former of which is generally considered wasteful to use on gems unless it is blessed to begin with

Before all that, there are easy ways of determining which gems are worth the effort of formally identifying.

Some valuable gems are supposed to only be generated at specific dungeon levels or below. In any case, all valuable gems will be approximately equally likely while the probability of worthless gems will stay unchanged. However, this is subject to bug C343-303: the gem probabilities are adjusted when a new level is generated, so e.g. level teleporting from level 1 to level 20 and back (assuming you had not been there before) will keep the gem probabilities from level 20. For this purpose, levels in branches other than Gehennom and the main dungeon always allow all gems.

These are the intended minimum levels for certain valuable gems:

Dungeon level

Gem

3

citrine stone

6

turquoise stone

9

emerald

12

black opal

15

sapphire

18

jacinth stone

21

ruby

24

diamond

27

dilithium crystal

Any other gems not listed above can be generated on any dungeon level.

Additionally, there are places where certain types of gems are guaranteed to be generated. Twelve gems – three diamonds (white), three emeralds (green), three rubies (red), and three amethysts (violet) – are guaranteed in the corner turrets of Fort Ludios, for example. Also, each bottom of the Gnomish Mines has several guaranteed gems, including at least an uncursedluckstone and an amethyst.

Amethysts have the special property of converting booze into fruit juice, which may be used to informally identify them (and also booze and fruit juice).

An uncursedtouchstone can easily separate glass from valuable gems. Even without one, there are ways of distinguishing valuable gems from glass. Some players type-name all gems to take advantage of the fact the hardness/color/price/is-glass quadruple is unique for each type of gem. The inventory list automatically creates stacks of different undefined gems that are described as the same color; there is only one pile of worthless glass of any given color. Next is hardness – all hard gems are valuable – and hardness can easily be tested by trying to engrave with gems on the dungeon floor. If you engrave in the dust, the gem is soft; if you scratch the floor itself, the gem is hard. Of the remaining groups of soft gems, you can throw one of each stack at a cross-alignedunicorn while utilizing naming tricks to keep track of which gems were found, which gems were to be thrown at the unicorn, and which gems were rejected by the unicorn. Cross-alignedunicorns only accept valuable gems "hesitatingly" and there is no Luck penalty for killing them, if without a Pet, though there may be an unknown Luck adjustment when they catch said valuable gems.

Do not bother to BUC identify your gems (not counting gray stones) at an altar. They are always generated uncursed and their BUC status has no effect. Marking them uncursed at an altar just causes the gems to fail to stack when you pick up more gems of the same type that haven't been formally BUC identified, wasting inventory slots. This can be particularly annoying if you are distinguishing valuable gems without formally identifying them, e.g. by throwing them at unicorns or using an uncursed touchstone, as it will prevent gems you pick up from stacking with named stacks.

Prices when unidentified

When selling gems which have not been formally identified to a shop, you will be offered a price from one of the "Unidentified sell price" columns. The values here will sometimes allow you to identify the gem. The column used is fixed for each shop (but may differ between shops), which may allow more gems to be identified if the column can first be determined.

The price table depends on compile time options. If you compiled NetHack yourself, check it is still valid.

SLASH'EM

Due to the existence of migohives, identifying four gems is slightly easier in SLASH'EM, due to the fact that only these gems are generated in migohives- the diamond, the ruby, the agate stone, and the fluorite stone. Hence, if you find a white gem in a migohive and can engrave with it, it's a diamond, and otherwise it's a fluorite stone. Blue, green, or purple gems found in migohives are also fluorite stones. If you find a red gem in a migohive, it's a ruby, and if you find an orange gem, it's an agate stone.

Alchemy with gems

Valuable gems can be dipped into a potion of acid to make new potions. See Alchemy.

Encyclopedia entries

gem or rock

The difference between false memories and true ones is the
same as for jewels: it is always the false ones that look the
most real, the most brilliant.
[ Salvador Dali ]

agate

Translucent, cryptocrystalline variety of quartz and a subvariety
of chalcedony. Agates are identical in chemical structure to
jasper, flint, chert, petrified wood, and tiger's-eye, and are
often found in association with opal. The colorful, banded rocks
are used as a semiprecious gemstone and in the manufacture of
grinding equipment. An agate's banding forms as silica from
solution is slowly deposited into cavities and veins in older
rock.
[ The Columbia Encyclopedia, Sixth Edition ]

amber

"Tree sap," Wu explained, "often flows over insects and traps
them. The insects are then perfectly preserved within the
fossil. One finds all kinds of insects in amber - including
biting insects that have sucked blood from larger animals."
[ Jurassic Park, by Michael Crichton ]

diamond

The hardest known mineral (with a hardness of 10 on Mohs' scale).
It is an allotropic form of pure carbon that has crystallized in
the cubic system, usually as octahedra or cubes, under great
pressure.
[ A Concise Dictionary of Physics ]

The diamond, _adamas_ or _dyamas_, is a transparent stone, like
crystal, but having the colour of polished iron, but it cannot
be destroyed by iron, fire or any other means, unless it is
placed in the hot blood of a goat; with sharp pieces of diamond
other stones are engraved and polished. It is no greater than
a small nut. There are six kinds, however Adamant attracts
metal; it expels venom; it produces amber (and is efficacious
against empty fears and for those resisting spells). It is
found in India, in Greece and in Cyprus, where magicians make
use of it. It gives you courage; it averts apparitions; it
removes anger and quarrels; it heals the mad; it defends you
from your enemies. It should be set in gold or silver and worn
on the left arm. It is likewise found in Arabia.
[ The Aberdeen Bestiary, translated by Colin McLaren ]

dilithium

The most famous and the first to be named of the imaginary
"minerals" of Star Trek is dilithium. ... Because of this
mineral's central role in the storyline, a whole mythology
surrounds it. It is, however, a naturally occurring substance
within the mythology, as there are various episodes that
make reference to the mining of dilithium deposits. ...
This name itself is imaginary and gives no real information on
the structure or make-up of this substance other than that this
version of the name implies a lithium and iron-bearing
aluminosilicate of some sort. That said, the real mineral that
most closely matches the descriptive elements of this name is
ferroholmquistite which is a dilithium triferrodiallosilicate.
If one goes on the premise that nature follows certain general
norms, then one could extrapolate that dilithium might have a
similar number of silicon atoms in its structure.
Keeping seven (i.e. hepto) ferrous irons and balancing the
oxygens would give a theoretical formula of Li2Fe7Al2Si8O27.
A mineral with this composition could theoretically exist,
although it is doubtful that it would possess the more fantastic
properties ascribed to dilithium.
[ The Mineralogy of Star Trek, by Jeffrey de Fourestier ]

emerald

'Put off that mask of burning gold
With emerald eyes.'
'O no, my dear, you make so bold
To find if hearts be wild and wise,
And yet not cold.'

'I would but find what's there to find,
Love or deceit.'
'It was the mask engaged your mind,
And after set your heart to beat,
Not what's behind.'

'But lest you are my enemy,
I must enquire.'
'O no, my dear, let all that be;
What matter, so there is but fire
In you, in me?'
[ The Mask, by W.B. Yeats ]

jacinth stone

Sweet in the rough weather
The voice of the turtle-dove-
'Beautiful altogether
Is my Love.
His Hands are open spread for love
And full of jacinth stones-
As the apple-tree among trees of the grove
Is He among the sons.'
[ The Beloved, by May Probyn ]

jade

Nothing grew among the ruins of the city. The streets were
broken and the walls of the houses had fallen, but there were
no weeds flowering in the cracks and it seemed that the city
had but recently been brought down by an earthquake. Only
one thing still stood intact, towering over the ruins. It
was a gigantic statue of white, gray and green jade - the
statue of a naked youth with a face of almost feminine beauty
that turned sightless eyes toward the north.
"The eyes!" Duke Avan Astran said. "They're gone!"
[ The Jade Man's Eyes, by Michael Moorcock ]

obsidian

A volcanic glass, homogeneous in texture and having a low water
content, with a vitreous luster and a conchoidal fracture. The
color is commonly black, but may be some shade of red or brown,
and cut sections sometimes appear to be green. Like other
volcanic glasses, obsidian is a lava that has cooled too quickly
for the contained minerals to crystallize. In chemical
composition it is rich in silica and similar to granite. It is
favored by primitive peoples for knives, arrowheads, spearheads,
and other weapons and tools.
[ The Columbia Encyclopedia, Sixth Edition ]

ruby and sapphire

_Corundum._ Mineral, aluminum oxide, Al2O3. The clear varieties
are used as gems and the opaque as abrasive materials. Corundum
occurs in crystals of the hexagonal system and in masses. It is
transparent to opaque and has a vitreous to adamantine luster.
[...] The chief corundum gems are the ruby (red) and the
sapphire (blue).
[ The Columbia Encyclopedia, Sixth Edition ]

topaz stone

Aluminum silicate mineral with either hydroxyl radicals or
fluorine, Al2SiO4(F,OH)2, used as a gem. It is commonly
colorless or some shade of pale yellow to wine-yellow;
[...] The stone is transparent with a vitreous luster. It has
perfect cleavage on the basal pinacoid, but it is nevertheless
hard and durable. The brilliant cut is commonly used. Topaz
crystals, which are of the orthorhombic system, occur in highly
acid igneous rocks, e.g., granites and rhyolites, and in
metamorphic rocks, e.g., gneisses and schists.
[ The Columbia Encyclopedia, Sixth Edition ]

turquoise stone

TUBAL: There came divers of Antonio's creditors in my company
to Venice that swear he cannot choose but break.
SHYLOCK: I am very glad of it; I'll plague him, I'll torture
him; I am glad of it.
TUBAL: One of them showed me a ring that he had of your
daughter for a monkey.
SHYLOCK: Out upon her! Thou torturest me, Tubal. It was my
turquoise; I had it of Leah when I was a bachelor; I would
not have given it for a wilderness of monkeys.
[ The Merchant of Venice, by William Shakespeare ]